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Abstract

The objective of this study was to determine the possible
application of ultrasonic inspection for non-destructive, online
evaluation of the integrity of heat sealed, flexible
package structure commonly used in packaging of aseptic and
shelf-stable food products.

A scanning acoustic microscope (SAM), Olympus UH-3, and
image analysis system were used to establish the operational
parameters to ultrasonically inspect the heat seal closure of
various flexible packages. The frequency range, attenuation,
and focal length (Z-value) were determined respectively for
paper laminate containers, plastic and plastic/aluminum
pouches and plastic trays with plastic or plastic/aluminum
lidding materials.

The SAM images of channel leakers, blisters and wrinkles
were sufficiently characteristic to allow their
identification. The same should be possible in an on-line,
ultrasonic testing device through proper design of the
transducers and scanning mechanism of the inspection system
and by monitoring of the ultrasonic signal. Channel leakers
of 20 Î¼m diameter were successfully detected in all package
structures with the exception of the paper laminate which
scattered the ultrasonic waves. The frequency used for
inspection ranged from 30 to 100 MHz and best results were
obtained when focussing at the seal bottom surface. As a
general rule, lower frequencies were used for inspection of
relatively thick seals or laminates containing an aluminum
layer. Geometry, thickness, surface characteristics, and
laminate composition of the seal to be inspected were found to
affect SAM's performance. The SAM was able to detect defects
as small as 20 Î¼m when working in the pulse mode, using
focussed transducers of frequency ranging between 30 and 100
MHz. However, because smaller defects could not technically
be manufactured at the seal interface, this value is not
definite and it is believed that smaller defects could be
successfully detected, especially in the higher frequency
range. For optimum results, seals to be inspected should be
free of embossment, flat, and should remain parallel to the
surface of the transducer during inspection. Finally, biotests
showed that a 20 Î¼m channel leaker in a seal of 5 mm
width was of sufficient size to cause post-process
contamination in Meal Ready to Eat (MRE, plastics/AI
structure) pouches.